Search

With rapid industrial development, a large amount of waste gases has been produced, causing severe industrial disasters and human health problems. In order to avoid gas-related accidents and prevent potential health issues, the detection and monitoring of hazardous gases is essential. In order to effectively detect harmful gases, semiconductor gas sensors have gained increasing attention due to their high sensitivity, small size, cost-effectiveness and ease of manufacturing. This article reviews metal oxide-based semiconductor gas sensors. Firstly, features of metal oxide-based semiconductor gas sensors including major advantages and limitations are discussed. Then, the operating mechanism of semiconductor gas sensors are discussed with a list of widely studied metal oxides. Finally, the semiconductor gas sensors made of four different metal oxides – i) tin oxide (SnO2), ii) indium oxide (In2O3), iii) zinc oxide (ZnO), and iv) tungsten trioxide (WO3) – are discussed from the aspects of current challenges, recent research strategies and future perspectives.

Metal complexes exhibiting multifunctional properties are very rare. Here, we report a Zn(II) complex, [Zn(benzoate)(25F-spy)2], where 25F-spy = 2′,5′-difluoro-4-styrylpyridine, displays unpredicte...

Stabilizing atomically dispersed single atoms (SAs) on Si photoanodes for photoelectrochemical-oxygen evolution reaction is still challenging due to the scarcity of anchoring sites. Here, we elaborately demonstrate the decoration of Ir SAs on Si photoanodes and assess the role of SAs on the separation and transfer of photogenerated charge carriers. NiO/Ni thin film, an active and highly stable catalyst, is capable of embedding the Ir SAs in its lattices by locally modifying the electronic structure. The isolated Ir SAs enable the effective photogenerated charge transport by suppressing the charge recombination and lower the thermodynamic energy barrier in the potential-determining step. The Ir SAs/NiO/Ni/ZrO2/n-Si photoanode exhibits a benchmarking photoelectrochemical performance with a high photocurrent density of 27.7 mA cm− 2 at 1.23 VRHE and 130 h stability. This study proposes the rational design of SAs on Si photoelectrodes and reveals the potential of the Ir SAs to boost photogenerated charge carrier kinetics.

The performance of halide perovskite-based electronic and optoelectronic devices is often related to interfacial charge transport. To shed light on the underlying physical and chemical properties of CH3NH3PbI3 (MAPbI3) in direct contact with common electrodes Al, Ti, Cr, Ag, and Au, the evolution of interfacial properties and Fermi level pinning is systematically studied. Given a unique experimental facility, pristine interfaces without any exposure to ambient air were prepared. We observe aggregation of substantial amounts of metallic lead (Pb0) at the metal/MAPbI3 interface, resulting from the interfacial reaction between the deposited metal and iodine ions from MAPbI3. It is found that the Schottky barrier height at the metal/MAPbI3 interface is independent of the metal work function due to strong Fermi level pinning, possibly due to the metallic Pb0 aggregates, which act as interfacial trap sites. The charge neutrality level of MAPbI3 is consistent with the energy level of Pb0-related defects, indicating that Pb0 interfacial trap states can be nonradiative recombination sites. This work underlines that control of chemical bonding at interfaces is a key factor for designing future halide perovskite-based devices.

Two-dimensional ferroelectrics is attractive for synaptic device applications because of its low power consumption and amenability to high-density device integration. Here, we demonstrate that tin monosulfide (SnS) films less than 6 nm thick show optimum performance as a semiconductor channel in an in-plane ferroelectric analogue synaptic device, whereas thicker films have a much poorer ferroelectric response due to screening effects by a higher concentration of charge carriers. The SnS ferroelectric device exhibits synaptic behaviors with highly stable room-temperature operation, high linearity in potentiation/depression, long retention, and low cycle-to-cycle/device-to-device variations. The simulated device based on ferroelectric SnS achieves ∼92.1% pattern recognition accuracy in an artificial neural network simulation. By switching the ferroelectric domains partially, multilevel conductance states and the conductance ratio can be obtained, achieving high pattern recognition accuracy.

Green hydrogen is being considered as a next-generation sustainable energy source. It is created electrochemically by water splitting with renewable electricity such as wind, geothermal, solar, and hydropower. The development of electrocatalysts is crucial for the practical production of green hydrogen in order to achieve highly efficient water-splitting systems. Due to its advantages of being environmentally friendly, economically advantageous, and scalable for practical application, electrodeposition is widely used to prepare electrocatalysts. There are still some restrictions on the ability to create highly effective electrocatalysts using electrodeposition owing to the extremely complicated variables required to deposit uniform and large numbers of catalytic active sites. In this review article, we focus on recent advancements in the field of electrodeposition for water splitting, as well as a number of strategies to address current issues. The highly catalytic electrodeposited catalyst systems, including nanostructured layered double hydroxides (LDHs), single-atom catalysts (SACs), high-entropy alloys (HEAs), and core-shell structures, are intensively discussed. Lastly, we offer solutions to current problems and the potential of electrodeposition in upcoming water-splitting electrocatalysts.

Summary Direct utilization of abundantly available solar energy is a promising way to create a sustainable society. Here, we report a ferroelectric-semiconductor (BaTiO3@BiVO4) hybrid that uses an effective strategy to enhance charge separation and transfer during water oxidation. The tetragonal BaTiO3 can induce an outward vector of built-in electric field after positive polarization, which aids in increasing the photovoltage and accelerating holes’ transfer to BiVO4’s surface. A super-hygroscopic metal hydrogel serves as an atmospheric humidity harvester for continuous water supply to the hybrid, where water oxidation takes place. As the hydrogel absorbs moisture from ambient humid air, it functions as a dehumidifying agent and carries water to the photoanode for power generation, being connected in series with a solar cell, further boosting the carrier’s mobility. This photoanode-hydrogel and solar-cell intelligent assembly can generate a photocurrent of 0.4 mA/cm2 with a relative humidity reduction of 12.0% under an illumination of 10 mW/cm2.

Hybrid organic-inorganic perovskites (HOIPs) are a new generation of high-performance materials for solar cells and light emitting diodes. Beyond these applications, ferroelectricity and spin-related properties of HOIPs are increasingly attracting interests. The presence of strong spin-orbit coupling, allied with symmetry breaking ensured by remanent polarization, should give rise to Rashba-type splitting of electronic bands in HOIP. However, the report of both ferroelectricity and Rashba effect in HOIP is rare. Here we report the observation of robust ferroelectricity and Rashba effect in two-dimensional Dion-Jacobson perovskites. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

Transition metal dichalcogenides (TMDs) have attracted enormous attention in diverse research fields. Especially, gas sensors are considered in a promising application exploiting TMDs. However, the...

To date, chemoresistive gas sensors based on metal oxide semiconductors (MOS) have been the most attractive sensor types for the practical application. However, with the emerging concept of Internet of Everything, high operating temperatures over 300 °C of the gas sensors based on MOS must be reduced to achieve low power consumption and overcome a limited battery capacity of mobile devices. The 2-dimensional materials like MoS2 have been, therefore, one of the most recently studied materials for gas sensors with capability of operation at significantly lower temperatures. However, lacking selectivity toward various target gas species limited their application to gas sensors. Herein, we investigated the effects of noble metals (Pd and Au) decoration on the gas sensing properties of MoS2 thin films. Due to the electronic sensitization of the noble metal catalysts and the formation of Pd hydride, overall gas responses and selectivity were significantly improved toward tests gas species. These results provide clear understandings on the effects of the surface noble metal catalysts on gas sensing properties of MoS2.

This paper investigates the gas sensing characteristics of the MOSFET-type sensor having an inkjet-printed WS2 sensing layer. The drain current of the gas sensor increases when NO2 gas is injected into the test chamber since NO2 gas is an oxidizing gas that extracts electrons from the sensing layer. On the contrary, the drain current decreases when H2S gas is injected into the test chamber since H2S gas is a reducing gas that donates electrons to the sensing layer. In both cases, the change of the drain current increases as the gas concentration increases. However, for other gases (NH3 and CO2), the gas sensor has a small change of the drain current. The responses of the gas sensor to 10 ppm NO2, H2S, NH3, and CO2 gases are 15.20%, 7.18%, 1.66%, and 3.02%, respectively. Therefore, the WS2 sensor has a high selectivity for NO2 gas among the four target gases.

In order to develop high performance chemoresistive gas sensors for Internet of Everything applications, low power consumption should be achieved due to the limited battery capacity of portable devices. One of the most efficient ways to reduce power consumption is to lower the operating temperature to room temperature. Herein, we report superior gas sensing properties of SnS2 nanograins on SiO2 nanorods toward NO2 at room temperature. The gas response is as high as 701% for 10 ppm of NO2 with excellent recovery characteristics and the theoretical detection limit is evaluated to be 408.9 ppb at room temperature, which has not been reported for SnS2-based gas sensors to the best of our knowledge. The SnS2 nanograins on the template used in this study have excessive sulfur component (Sn:S = 1:2.33) and exhibit p-type conduction behavior. These results will provide a new perspective of nanostructured two-dimensional materials for gas sensor applications on demand.

Two-dimensional MoS2 film can grow on oxide substrates including Al2O3 and SiO2. However, it cannot grow usually on non-oxide substrates such as a bare Si wafer using chemical vapor deposition. To address this issue, we prepared as-synthesized and transferred MoS2 (AS-MoS2 and TR-MoS2) films on SiO2/Si substrates and studied the effect of the SiO2 layer on the atomic and electronic structure of the MoS2 films using spherical aberration-corrected scanning transition electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The interlayer distance between MoS2 layers film showed a change at the AS-MoS2/SiO2 interface, which is attributed to the formation of S–O chemical bonding at the interface, whereas the TR-MoS2/SiO2 interface showed only van der Waals interactions. Through STEM and EELS studies, we confirmed that there exists a bonding state in addition to the van der Waals force, which is the dominant interaction between MoS2 and SiO2. The formation of S–O bonding at the AS-MoS2/SiO2 interface layer suggests that the sulfur atoms at the termination layer in the MoS2 films are bonded to the oxygen atoms of the SiO2 layer during chemical vapor deposition. Our results indicate that the S–O bonding feature promotes the growth of MoS2 thin films on oxide growth templates.

Dion-Jacobson (DJ) phase organic-inorganic hybrid perovskites (OIHPs) have emerged as promising alternatives to Ruddlesden-Poppers perovskites because of their chemical stability and ferroelectric phase. Here we fabricate a ferroelectricity-modulated photodetector based on the n = 2 homologue of the ferroelectric two-dimensional DJ-OIHP (AMP)(MA)Pb2I7 (DJPn=2, AMP = 4-(aminomethyl)piperidinium; MA = methylammonium), which shows an out-of-plane polarization and a saturated polarization (Ps) value of 3.7 μC/cm2. The coercive field of DJPn=2 (0.34 kV/cm) is lower than that for the n = 1 homologue (AMP)PbI4 (DJPn=1,0.4 kV/cm). DJPn=2 has a much longer carrier lifetime and absorption edge (580 nm, 2.13 eV) in comparison to DJPn=1 (523 nm, 2.37 eV); thus, DJPn=2 can be used for efficient photodetection in the visible range, in which a responsivity of 0.16 mA/W was achieved at 532 nm. The influence of remnant polarization on the direction and magnitude of the photocurrent was also demonstrated.

Vertically ordered hematite nanotubes decorated with Au nanoparticles are synthesized by a facile electrochemical anodization method and self-agglomeration of Au films. Au decoration significantly enhances the response of hematite nanotubes to acetone at 350 °C and the response to acetone is 2.4 times higher than the response to ethanol, the counterpart target gas. The Au nanoparticles decorated hematite nanotubes exhibit unprecedentedly high and selective responses to acetone at 350 °C with 50% relative humidity condition (calibrated at room temperature, 25 °C). The enhanced gas sensing properties in a humid condition were attributed to the spillover effect by Au nanoparticles and high porosity of hematite nanotubes with a large surface-to-volume ratio. These results indicate that Au nanoparticles decorated hematite nanotubes are promising for use in high quality sensor materials for breath analyzers to diagnose diabetes mellitus.

Despite its abundant, nontoxicity and photochemical stability, titanium dioxide shows low solar water oxidation performance due to low photogenerated carrier transport and wide optical band gap, which results in substantially low photogenerated carrier density that impair the solar to hydrogen conversion efficiency. Herein, highly enhanced water oxidation performance of high-aspect-ratio TiO2 nanorods doped with dual heteroatoms, sulfur and nitrogen, for photoelectrochemical solar water oxidation is demonstrated. The codoped TiO2 NRs have shown enhanced optical absorption coefficient due to the induced impurities energy states near to the top of the valance band and result in a red shift in the optical absorption edges. Consequently, a 2.82 mAcm−2 photocurrent density at 1.23 V vs. RHE is obtained from the sulfur and nitrogen codoped TiO2 nanorods, and pristine TiO2 nanorods photoanode shows 0.7 mAcm−2. The applied bias photon-to-current conversion efficiency and external quantum efficiency of the codoped TiO2 nanorods are 1.49% and 97.0% at λ = 360 nm and 0.69% and 19.1% at λ = 370 nm for pristine TiO2 nanorods, respectively. Our study offers experimental and theoretical evidence for codoping of sulfur and nitrogen improve the optical and electrical properties of TiO2 for efficient photoelectrochemical solar water oxidation.

The utilization of edge sites in two-dimensional materials including transition-metal dichalcogenides (TMDs) is an effective strategy to realize high-performance gas sensors because of their high catalytic activity. Herein, we demonstrate a facile strategy to synthesize the numerous edge sites of vertically aligned MoS2 and larger surface area via SiO2 nanorod (NRs) platforms for highly sensitive NO2 gas sensor. The SiO2 NRs encapsulated by MoS2 film with numerous edge sites and partially vertical-aligned regions synthesized using simple thermolysis process of [(NH4)2MoS4]. Especially, the vertically aligned MoS2 prepared on 500 nm thick SiO2 NRs (500MoS2) shows approximately 90 times higher gas-sensing response to 50 ppm NO2 at room temperature than the MoS2 film prepared on flat SiO2, and the theoretical detection limit is as low as ∼2.3 ppb. Additionally, it shows reliable operation with reversible response to NO2 gas without degradation at an operating temperature of 100 °C. The use of the proposed fa...

Two-dimensional transition-metal dichalcogenides (TMDCs) are very promising for photovoltaic (PV) applications due to their excellent light absorption properties and appropriate bandgap energy, Although multifunctional applications of TMDCs in photovoltaic devices have been achieved, the photovoltaic conversion efficiency under 1 sun is still very low with small active area because of their inexpedient high sheet resistance and limitation of synthesis techniques. In this study, we demonstrate uniform synthesis of 4-in. wafer-scale MoS2 thin films by thermal decomposition of solution precursors. The solar cells are fabricated by transferring n-MoS2 thin films on p-Si substrates to form p-n heterojunctions and then transferring Au nanomeshes prepared in a novel surface treatment as transparent top electrodes onto MoS2. The circular n-MoS2/p-Si heterojunction solar cell exhibited a power conversion efficiency of 5.96% at a diameter of 0.3 in. and proved to be easily scalable to 1-in. diameter with 5.18% efficiency. To the best of our knowledge, the solar cells of this study are the most efficient and the largest in all types of solar cells based on TMDC reported so far. Finally, based on finite difference time-domain simulation, we proposed a strategy for implementing n-MoS2/p-Si heterojunction solar cell with efficiency higher than 15% by introducing optimal doping control of n-MoS2 and efficient anti-reflection layers.

The preparation of crystalline materials on incommensurate substrates has been a key topic of epitaxy. van der Waals (vdW) epitaxy on two-dimensional (2D) materials opened novel opportunities of epitaxial growth overcoming the materials compatibility issue. Therefore, vdW epitaxy has been considered as a promising approach for the preparation of building blocks of flexible devices and thin film-based devices at the nano/microscale. However, an understanding of vdW epitaxy has not been thoroughly established. Especially, controlling nucleation during vdW epitaxy has not been achieved although nucleation in vdW epitaxy is suppressed due to the absence of surface dangling bonds on 2D materials. Here we show an enhancement of nucleation probability of germanium on graphene via introducing an out-of-plane dipole moment without any change in the chemical nature of graphene. A graphene/hexagonal boron nitride stack and transferred graphene on a polarized ferroelectric thin film were employed to demonstrate the significant enhancement of Ge nucleation on graphene. Theoretical calculations and chemical vapor deposition were employed to elucidate the effect of the out-of-plane dipole moment on nucleation in vdW epitaxy.

Reduced graphene oxide (rGO) and MoS2, the most representative two-dimensional (2D) materials, are receiving significant attention for the fabrication of sensing devices owing to their high surface area, many abundant sites, and excellent mechanical flexibility. Herein, we demonstrated chemoresistive humidity sensors with fast response, excellent selectivity, and ultrahigh sensitivity based on 2D rGO and 2D MoS2 hybrid composites (RGMSs). The RGMSs were fabricated by simple ultrasonication without the addition of additives and additional heating. Compared to pristine rGO, the RGMS exhibited a 200 times higher response to humidity at room temperature. The significant enhancement in the sensing performance of the composite was attributed to electronic sensitization due to p–n heterojunction formation and porous structures between rGO and MoS2. The synergistic combination of rGO and MoS2 could be applied to construct a flexible humidity sensor. The sensing performance of an RGMS flexible device remains the same before and after bending; this indicates that the use of rGO and MoS2 is a viable new and simple strategy to realize a humidity sensor for use in wearable electronics.

Transferable 2-dimensional (2D) MoS2 thin films have a versatile potential for constructing highly efficient photoelectrodes when combined with conventional semiconductor light absorbers, taking advantage of its optical transparency and high electrochemical activity. Here, we firstly report fully vertically aligned MoS2 (VMS)/p-Si heterostructure photocathode for photoelectrochemical (PEC) water splitting. Furthermore, 3D iron-nickel sulfide nanoparticles of tailored atomic composition are formed simultaneously during the synthesis of VMS via one-step sulfurization to build 3D/2D transition metal sulfide (TMD) heterostructure thin film catalyst. The spectroscopic results reveal that the Fe-doped Ni3S2 nanoparticles on VMS/p-Si photocathode induce the electrochemically-benign band bending in the overall heterostructure, enabling a significant improvement in PEC performance and long-term stability. Scanning photoelectrochemical microscopy is used to vividly visualize the photocurrent enhancement by the various 3D/2D TMD heterostructures. This work provides promising strategies in developing high performance TMD-based electrocatalysts for practical applications in a wide variety of electrochemical energy conversion processes.

It was demonstrated that organolead halide perovskites (OHPs) show a resistive switching behavior with an ultralow electric field of a few kilovolts per centimeter. However, a slow switching time and relatively short endurance remain major obstacles for the realization of the next-generation memory. Here, we report a performance-enhanced OHP resistive switching device. To fabricate topologically and electronically improved OHP thin films, we added hydroiodic acid solution (for an additive) in the precursor solution of the OHP. With drastically improved morphology such as small grain size, low peak-to-valley depth, and precise thickness, the OHP thin films showed an excellent performance as insulating layers in Ag/CH3NH3PbI3/Pt cells, with an endurance of over 103 cycles, a high on/off ratio of 106, and an operation speed of 640 μs and without electroforming. We suggest plausible resistive switching and conduction mechanisms with current–voltage characteristics measured at various temperatures and with dif...

Development of clean and renewable energy to attain a sustainable society is one of the most urgent tasks at present. In this respect, the production of hydrogen (H 2 ) through electrochemical or photoelectrochemical water splitting is a promising way to utilize sustainable energy source such as sunlight. Recently, transition metal disulfides (TMDs) have received tremendous attention as hydrogen evolution reaction (HER) catalysts due to their favorable chemical and catalytic properties. Theoretical and experimental demonstrations have revealed that TMD-based catalysts exhibit excellent catalytic activity at edge sites. However, there is limitation that the basal plane which constitutes most of the TMD material remains chemically inert. Herein, we overview recent progress in tailoring catalytic activities of TMDs for electrochemical and photoelectrochemical hydrogen production and discuss the future strategies to overcome the conventional limitation of TMD-based catalysts.

We demonstrate the tungsten disulfide (WS2) thin film catalysts prepared by the sulfurization of vacuum deposited WO3 thin films for efficient hydrogen production with over 90% Faradaic efficiency. The 23-nm-thick WS2 thin film catalyst heterojunction with p-type silicon photocath-ode could exhibit a photocurrent density of 8.3 mA/cm2 at 0 V versus a reversible hydrogen electrode (RHE), a low onset potential of 0.2 V versus RHE when photocurrent density reaches -1 mA/cm2 and long-term stability over 10 h. The enhanced catalytic activities of WS2/p-Si photocathodes compared with the bare p-Si photocathode originate from a number of edge sites in the synthesized polycrystalline thin films, which could act as hydrogen evolution catalyst.

The twisted nematic liquid crystal cell was developed by using a CYTOP-transferred graphene sheet as an electrode and an alignment layer. A graphene layer was synthesized by chemical vapor deposition and transferred onto a plastic substrate using a fluoropolymer known as CYTOP. As the ion-beam treatment time increased, the sheet resistance increased from 500 to 1100 Ω/sq., while the water contact angle decreased from 110.5° to 69.7°. The increased intensities of the D and G′ bands and the appearance of D + D″ and D + G′ bands in the Raman spectra indicated the formation of defects because of the ion-beam treatment. An ion-beam exposure time of 15 s was found to be the most effective for the production of CYTOP-transferred graphene and for achieving high contrast in operating cells. The ion beam detached F from the CYTOP-transferred graphene layer, and the resulting exposure of the C=C bond on the graphene surface affected the alignment of liquid crystal molecules. Based on these results, the technique described here has applications in novel, high-performance liquid crystal displays that do not require indium-tin-oxide electrodes and polyimide alignment layers. Sheets synthesized by chemical vapor deposition were transferred and simultaneously doped using fluoropolymer supporting layers.

Various noble metal-free electrocatalysts have been explored to enhance the overall water splitting efficiency. Ni-based compounds have attracted substantial attention for achieving efficient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalysts. Here, we show superior electrocatalysts based on NiFe alloy electroformed by a roll-to-roll process. NiFe (oxy)hydroxide synthesized by an anodization method for the OER catalyst shows an overpotential of 250 mV at 10 mA cm-2, which is dramatically smaller than that of bare NiFe alloy with an overpotential of 380 mV at 10 mA cm-2. Electrodeposited NiMo films for the HER catalyst also exhibit a small overpotential of 100 mV at 10 mA cm-2 compared with that of bare NiFe alloy (550 mV at 10 mA cm-2). A combined spectroscopic and electrochemical analysis reveals a clear relationship between the surface chemistry of NiFe (oxy)hydroxide and the water splitting properties. These outstanding fully solution-processed catalysts facilitate superb overall water splitting properties due to enlarged active surfaces and highly active catalytic properties. We combined a solution-processed monolithic perovskite/Si tandem solar cell with MAPb(I0.85Br0.15)3 for the direct conversion of solar energy into hydrogen energy, leading to the high solar-to-hydrogen efficiency of 17.52%. Based on the cost-effective solution processes, our photovoltaic-electrocatalysis (PV-EC) system has advantages over latest high-performance solar water splitting systems.

Transition metal dichalcogenides (TMDs) have attracted enormous attention in diverse research fields. Especially, gas sensors are considered in a promising application exploiting TMDs. However, the studies are confined to only major TMDs such as MoS

In order to develop high performance chemoresistive gas sensors for Internet of Everything applications, low power consumption should be achieved due to the limited battery capacity of portable devices. One of the most efficient ways to reduce power consumption is to lower the operating temperature to room temperature. Herein, we report superior gas sensing properties of SnS

One of the well-known pathways toward low power consuming chemoresistive gas sensors is the utilization of 2-dimensional materials. Especially, transition metal dichalcogenides (TMDs), which are usually atomically thin semiconductors, have a notable characteristic of their highly reactive edge sites. The edge sites of TMDs having high d-orbital electron density can serve as highly favorable chemically active sites for direct interaction with target gas molecules. In this study, WS2 was synthesized on highly porous SiO2 nanorods template to have numerous edge-exposed WS2 flakes in a limited active area taking advantage of 1-dimensional nanostructures with extremely high surface-to-volume ratio. The fabricated WS2 on 1D nanostructures exhibited a gas response of 151.2 % toward 5 ppm NO2, which has not been reported in performance-wise at room temperature to the best of the author’s knowledge. Density functional theory calculations theoretically supported the highly sensitive and selective NO2 detection with a theoretical detection limit of 13.726 ppb.

Two-dimensional transition metal dichalcogenides (TMDs) have been intensively researched due to their excellent physical, chemical, and mechanical properties, which make them essential for various electronic devices owing to several disadvantages of conventional hole-transport layers (HTLs) such as hygroscopic effect and highly acidic nature, which can induce low stability of the fabricated devices. Especially, they have been considered as hole-transport layers (HTLs) in organic light-emitting diodes (OLEDs) and organic photovoltaics due to its chemical stability. Despite of its adequate work-function value and chemical stability, the device stability could be enhanced but, device performance of pristine TMD-HTL-based has been reported lower than conventional devices. In this work, we report a facile route to synthesize alloyed transition metal disulfides (TMD) thin films and their application as hole transport layers in OLEDs. Polycrystalline, large-area, and uniform Mo1−XWXS2 thin films are synthesized via simple thermal disproportionation methods by chemical vapour deposition. The physical and chemical properties of the synthesized alloyed TMD layers are controlled by varying the precursor concentrations. The device performance of alloyed TMD-layer-based OLED is comparable to that of conventional poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) and device stability in air is significantly improved. Thus, a novel approach to synthesize alloyed Mo1−XWXS2 thin films and their application in optoelectronic devices are presented herein.

We synthesized transferrable and transparent anion-engineered molybdenum disulfide thin-film catalysts through a simple thermolysis method by using [(NH4)2MoS4] solution and powder precursors with different sulphur/phosphorus weight ratios. The synthesized sulphur-doped molybdenum phosphide (S:MoP) thin film changed from a two-dimensional van der Waals structure to a three-dimensional hexagonal structure by introduction of phosphorus atoms in the MoS2 thin film. The S:MoP thin film catalyst, which is composed of cheap and earth abundant elements, could provide the lowest onset potential and the highest photocurrent density for planar p-type Si photocathodes. The density functional theory calculations indicate that the surface of S:MoP thin films absorb hydrogen better than that of MoS2 thin films. The structurally engineered thin film catalyst facilitates the easy transfer of photogenerated electrons from the p-Si light absorber to the electrolyte. Anion-engineering of the MoS2 thin film catalyst would be an efficient way to enhance the catalytic activity for photoelectrochemical water splitting.

Vertically ordered hematite nanotubes are considered to be promising photoactive materials for high-performance water-splitting photoanodes. However, the synthesis of hematite nanotubes directly on conducting substrates such as fluorine-doped tin oxide (FTO)/glass is difficult to be achieved because of the poor adhesion between hematite nanotubes and FTO/glass. Here, we report the synthesis of hematite nanotubes directly on FTO/glass substrate and high-performance photoelectrochemical properties of the nanotubes with NiFe cocatalysts. The hematite nanotubes are synthesized by a simple electrochemical anodization method. The adhesion of the hematite nanotubes to the FTO/glass substrate is drastically improved by dipping them in nonpolar cyclohexane prior to postannealing. Bare hematite nanotubes show a photocurrent density of 1.3 mA/cm(2) at 1.23 V vs a reversible hydrogen electrode, while hematite nanotubes with electrodeposited NiFe cocatalysts exhibit 2.1 mA/cm(2) at 1.23 V which is the highest photocurrent density reported for hematite nanotubes-based photoanodes for solar water splitting. Our work provides an efficient platform to obtain high-performance water-splitting photoanodes utilizing earth-abundant hematite and noble-metal-free cocatalysts.

Atomically thin two-dimensional materials such as MoS2, WS2, and graphene oxide (GO) are used as hole extraction layers (HEL) in organolead halide perovskites solar cells (PSCs) instead of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HEL. MoS2 and WS2 layers with a polycrystalline structure were synthesized by a chemical deposition method using a uniformly spin-coated (NH4)MoS4 and (NH4)WS4 precursor solution. GO was synthesized by the oxidation of natural graphite powder using Hummers' method. The work functions of MoS2, WS2, and GO are measured to be 5.0, 4.95, and 5.1 eV, respectively. The X-ray diffraction spectrum indicated that the synthesized perovskite material is CH3NH3PbI3−xClx. The PSCs with the p-n junction structure were fabricated based on the CH3NH3PbI3−xClx perovskite layer. The power conversion efficiencies of the MoS2, WS2, and GO-based PSCs were 9.53%, 8.02%, and 9.62%, respectively, which are comparable to those obtained from PEDOT:PSS-based devices (9.93%). These results suggest that two-dimensional materials such as MoS2, WS2, and GO can be promising candidates for the formation of HELs in the PSCs.

Graphene has shown strong potential to occupy transparent electrodes, replacing indium tin oxide (ITO). However, the commercialization of graphene is still limited because of its poor chemical and electrical stability from reaction with environmental factors or essential materials such as poly[3,4-(ethylenedioxy)thiophene]:poly(styrenesulfonate) (PSS). Here, we have demonstrated a multilayered electrode in which graphene is sandwiched between metal oxides (MOs) that have high stability and optical properties. The MOs overcoated graphene, and thereby protected it from desorption of chemical dopants. Because of the resulting chemical and electrical stability, the electrodes maintain low sheet resistance 2.4 times longer than bare graphene and 36 times longer thanPSS-coated graphene. On the basis of optical simulations, we derive the design rules for highly transparent MO/graphene/MO stacks and demonstrate an optimized structure with a TiO2 and WO3 electrode that has high transmittance (96%) which exceeds those of ITO (87%) and graphene (90%). Using a TiO2/graphene/WO3 electrode in organic light-emitting diodes (λ = 520 nm) instead of ITO or graphene anodes increases the cavity resonance and thereby increases power efficiencies by up to 30%. The MO/graphene/MO stacks designed will provide opportunities for commercialization of flexible electronics with graphene electrodes.

Organolead halide perovskites (OHPs) have attracted extensive attention as light harvesting materials for solar cells recently, because of their high charge carrier mobility, high photoconversion efficiencies, low cost, and simple methodology. Despite these advantages, the OHPs exhibit sweep-dependent hysteresis behavior in current–voltage characteristics films, deteriorating the reliability of devices based on the OHPs. This study demonstrates reliable high on/off ratio (Ion/Ioff = 104) CH3NH3PbI3 broadband photodetectors with buffer layer-free simple metal/semiconductor/metal lateral structure. At high external bias, poor on/off ratios and spikes in dark current and photocurrent are observed due to the migration of charged defect ions. The ion migration can be effectively inhibited at low external bias, and thus the devices show high Ion/Ioff ratios and spike-free dark current and photocurrent. In addition, prevention of the prepoling in the CH3NH3PbI3 films by operating at the low external bias results in pronouncedly enhanced signal-to-noise ratios even under low intensity incident light. These results strongly propose that inhibiting the migration of charged defect ions in CH3NH3PbI3 films is a key in developing reliable high performance CH3NH3PbI3-based devices.

Two-dimensional transition metal disulfide (TMD) nanosheets, including MoS2, WS2, TaS2, and TiS2, were used to catalyze the hydrogen evolution reaction (HER). The TMDs were exfoliated by sonication to generate nanosheet layers that were approximately a few hundred nanometers in size. X-ray diffraction and transmission electron microscope data indicated that the major plane of the exfoliated nanosheets was the (002) plane and that the hexagonal structure is maintained after exfoliation with lattice constants of 0.32 nm for MoS2 and WS2 and 0.34 nm for TaS2 and TiS2. Exfoliated MoS2, WS2, TaS2, and TiS2 loaded on Au electrodes exhibited good electrocatalytic activity with low onset potentials of ∼100, 150, 175, and 135 mV, respectively, at a current density of −1 mA/cm2. MoS2 and TiS2 exhibited the best HER performance with Tafel slopes of 94.91 and 91 mV/decade. These results indicated that TMD nanosheets have potential applications as HER catalysts for the mass production of hydrogen.

The utilization of edge sites in two-dimensional materials including transition-metal dichalcogenides (TMDs) is an effective strategy to realize high-performance gas sensors because of their high catalytic activity. Herein, we demonstrate a facile strategy to synthesize the numerous edge sites of vertically aligned MoS